Post-Total Thyroidectomy Tetany - ENT PG Exam Notes

1. DEFINITION AND INCIDENCE

• Transient symptomatic hypocalcemia occurs in 7-25% of cases after total thyroidectomy
• Permanent hypocalcemia (>6 months): 0.4-13.8%
• Risk rises to ~20% after combined thyroidectomy + radical neck dissection
• Symptoms typically manifest within the first 24-96 hours (usually within first 5 days) post-operatively

2. PHYSIOLOGICAL BASIS - CALCIUM HOMEOSTASIS

Normal Calcium Physiology

• Normal serum calcium: 8.5-10.2 mg/dL
• Exists in two forms:
  • Bound to protein (55%) - mainly albumin; this fraction is physiologically inert
  • Free ionized calcium (45%) - the biologically active form; normal range 4.5-5.0 mg/dL
• Key rule: For every 1.0 mg/dL decrease in albumin, there is a 0.8 mg/dL decrease in total calcium (corrected calcium formula)
• Ionized calcium is decreased by 0.36 mmol/L for every 1 unit increase in pH - alkalosis worsens tetany by reducing ionized calcium

PTH - The Master Regulator

• PTH is an 84-amino acid protein with a half-life of only 3-5 minutes
• Secreted in response to low serum ionized calcium (negative feedback when calcium is high)
• End organs of PTH action: kidneys, intestines, bone

• Calcitonin (from thyroid parafollicular C cells) inhibits bone resorption but plays a minor role in overall calcium regulation - even in medullary thyroid carcinoma with very high calcitonin, patients do NOT develop hypocalcemia

3. MECHANISM OF POST-THYROIDECTOMY TETANY

Primary Mechanism: Devascularization (Most Common)

Post-operative hypoparathyroidism is most frequently caused by disruption of the fine vessels that supply the parathyroids, rather than by accidental removal of all glands.

• Scott-Brown's Otorhinolaryngology Head & Neck Surgery

• Renal tubular calcium reabsorption decreases
• 1,25-(OH)2D3 synthesis falls → intestinal calcium absorption decreases
• Osteoclastic bone resorption decreases
• Net result: serum ionized calcium falls

Why Ionized Calcium Causes Tetany (Physiological Mechanism)

• Ionized calcium normally stabilizes the voltage-gated sodium channels on neuronal membranes
• In hypocalcemia: reduced extracellular Ca2+ lowers the threshold for nerve excitation (the channels open more easily)
• This results in spontaneous, repetitive depolarization of peripheral nerves and muscles
• The result: tetany, carpopedal spasm, laryngospasm, bronchospasm, and seizures

Contributing Factors

1. Alkalosis (e.g., from hyperventilation post-op) - reduces ionized calcium further by shifting equilibrium toward bound form
2. Hypoalbuminemia (post-op hemodilution) - though total Ca may fall, ionized Ca may be relatively preserved
3. Vitamin D deficiency pre-operatively - greatly increases the risk of severe post-op hypocalcemia
4. Hungry bone syndrome - rapid uptake of calcium into bones post-operatively (especially after parathyroid surgery or thyrotoxicosis correction)
5. Hypomagnesemia (<0.8 mEq/L) - suppresses PTH secretion AND interferes with PTH action at end organs

Timing

• PTH half-life is 3-5 minutes, so PTH levels fall quickly after parathyroid devascularization
• However, there is often a lag of 24-36 hours before serum calcium starts to drop (due to calcium stored in bone continuing to release)

4. CLINICAL FEATURES - IN SEQUENCE

Early (Latent Tetany)

• Tingling and numbness of lips, nose, and fingertips (perioral and acral paresthesias)
• Anxiety, restlessness

Established

• Carpopedal spasm: strong adduction of thumbs + extension of feet ("Obstetrician's hand" - fingers extended, MCP joints slightly flexed, thumb strongly adducted)
• Painful cramps of hands and feet
• Blurring of vision (spasm of intraocular muscles)

Severe / Late

• Generalized muscle cramps spreading to all muscles
• Laryngospasm / stridor / apnea - most dangerous acute complication
• Bronchospasm
• Seizures
• Papilledema
• Cardiac arrhythmias, prolonged QT interval on ECG, congestive heart failure
• Death (in untreated severe cases)

5. CLINICAL SIGNS (HIGH-YIELD FOR EXAMS)

Chvostek-Weiss Sign

• A gentle tap over the facial nerve as it emerges in front of the external auditory meatus (through the parotid gland)
• Positive: brisk muscular twitch of the same side of the face
• Less specific for hypocalcemia (can be positive in 10-25% of normal subjects)

Trousseau's Sign

• Sphygmomanometer cuff inflated around the arm to 200 mmHg (or above systolic) for 3-5 minutes
• Positive: typical "Obstetrician's hand" - fingers extended, MCP slightly flexed, thumb strongly adducted = carpopedal spasm
• More specific than Chvostek's sign for hypocalcemia

Monitoring Parameters Indicating Hypoparathyroidism

• Hypocalcemia
• Hyperphosphatemia (PTH normally promotes phosphate excretion)
• Metabolic alkalosis

6. MANAGEMENT OF POST-THYROIDECTOMY TETANY

Monitoring Protocol (Post-op)

• Serum calcium checked in immediate post-op period and the next morning after total thyroidectomy
• PTH level should be checked if hypoparathyroidism is suspected - helps predict hypocalcemia before levels drop (PTH falls before calcium does)
• Patient should have a stable or rising calcium before discharge
• Thyroid lobectomy patients do NOT require routine calcium monitoring

Treatment - Step by Step

• 10 mL of 10% calcium gluconate in 5% dextrose water, IV, given slowly, titrated to symptom resolution
• If severe tetany/seizures: IV calcium gluconate infusion at 0.5-1.5 mg/kg/hour elemental calcium
• If tetany causes respiratory compromise: chemical paralysis and intubation may be required
• Cardiac monitoring is warranted in severe cases

• Calcium carbonate: 2-3 g/day orally
• Calcitriol (1,25-dihydroxycholecalciferol): initiate early - bypasses the need for PTH-dependent renal hydroxylation
  • Note: Alfacalcidol is a prodrug requiring hepatic activation; impaired in liver disease
  • Calcitriol is preferred as it is already fully active

• Correct hypomagnesemia first (if present) - hypocalcemia will not respond to calcium replacement until magnesium is corrected
• Correct hyperphosphatemia: low-phosphate diet, oral phosphate-binding resins (aluminum hydroxide), saline infusion to increase urinary phosphate excretion

7. MECHANISMS TO AVOID POST-THYROIDECTOMY TETANY (Surgical Prevention)

1. Surgical Technique - Capsular Dissection

• Dissect close to the thyroid capsule to preserve parathyroid blood supply
• Ligate branches of the inferior thyroid artery on the thyroid capsule, distal to their supply of the parathyroid glands
• Do NOT ligate the inferior thyroid artery as a single trunk - this devascularizes all parathyroids at once
• Scott-Brown's: "The risk of accidental devascularization can be reduced by dissecting close to the thyroid capsule and ligating the branches of the inferior thyroid artery on the thyroid capsule distal to their supply of the parathyroid glands"

2. Parathyroid Autotransplantation

• Indicated when: central neck dissection required, en bloc resection, or reoperation after previous thyroid/parathyroid surgery
• Devascularized or inadvertently removed parathyroid tissue is minced and implanted into the sternocleidomastoid muscle (or brachioradialis of nondominant forearm)
• In MEN type 1: all parathyroids removed but half a gland autotransplanted into SCM or forearm to avoid severe hypocalcemia
• Cummings Otolaryngology: "Autotransplantation is commonly performed in the sternocleidomastoid muscle"

3. Pre-operative Vitamin D Optimization

• Correct vitamin D deficiency before surgery
• Correcting pre-op vitamin D deficiency reduces both the incidence of post-op hypocalcemia and the length of hospital stay
• Target serum 25-OH vitamin D > 50 nmol/L before surgery

4. Intraoperative PTH Monitoring

• Intraoperative PTH levels can predict post-op hypocalcemia
• PTH half-life of 3-5 minutes allows rapid intraoperative assessment

5. Surgeon Experience

• Risk is directly related to surgeon experience and case volume

8. EXAM MEMORY AIDS